Single-Mask Phase Change Memory Element

ABSTRACT

A memory device. An array of memory elements is formed on a semiconductor chip. A parallel array of word lines extends in a first direction, connecting each memory element to a data source, and a parallel array of bit lines extends in a second direction, connecting each memory element to a data source, the second direction forming an acute angle to the first direction. The connection between each bit line and each memory element is a phase change element composed of memory material having at least two solid phases.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to non-volatile memory structures, and more specifically to memory arrays employing phase change memory elements.

2. Description of Related Art

Phase change based memory materials are widely used in read-write optical disks and non-volatile memory arrays. These materials have at least two solid phases, including for example a generally amorphous solid phase and a generally crystalline solid phase. Laser pulses are used in read-write optical disks to switch between phases and to read the optical properties of the material after the phase change.

Phase change based memory materials, such as chalcogenide based materials and similar materials, also can be caused to change phase by application of electrical current at levels suitable for implementation in integrated circuits. The generally amorphous state is characterized by higher resistivity than the generally crystalline state, which can be readily sensed to indicate data. These properties have generated interest in using programmable resistive material to form nonvolatile memory circuits, which can be read and written with random access.

The change from the amorphous to the crystalline state is generally a lower current operation. The change from crystalline to amorphous, referred to as reset herein, is generally a higher current operation, which includes a short high current density pulse to melt or breakdown the crystalline structure, after which the phase change material cools quickly, quenching the phase change process, allowing at least a portion of the phase change structure to stabilize in the amorphous state. It is desirable to minimize the magnitude of the reset current used to cause transition of phase change material from crystalline state to amorphous state. The magnitude of the reset current needed for reset can be reduced by reducing the size of the phase change material element in the cell and of the contact area between electrodes and the phase change material, so that higher current densities are achieved with small absolute current values through the phase change material element.

One direction of development has been toward forming small pores in an integrated circuit structure, and using small quantities of programmable resistive material to fill the small pores. Patents illustrating development toward small pores include: Ovshinsky, “Multibit Single Cell Memory Element Having Tapered Contact,” U.S. Pat. No. 5,687,112, issued Nov. 11, 1997; Zahorik et al., “Method of Making Chalogenide [sic] Memory Device,” U.S. Pat. No. 5,789,277, issued Aug. 4, 1998; Doan et al., “Controllable Ovonic Phase-Change Semiconductor Memory Device and Methods of Fabricating the Same,” U.S. Pat. No. 6,150,253, issued Nov. 21, 2000.

Problems have arisen in manufacturing such devices with very small dimensions, and with variations in process that meets tight specifications needed for large-scale memory devices. It is desirable therefore to provide a memory cell structure having small dimensions and low reset currents, and a method for manufacturing such structure that meets tight process variation specifications needed for large-scale memory devices. It is further desirable to provide a manufacturing process and a structure, which are compatible with manufacturing of peripheral circuits on the same integrated circuit.

SUMMARY OF THE INVENTION

An important aspect of the claimed invention is a memory device. An array of memory elements is formed on a semiconductor chip. A parallel array of word lines extends in a first direction, connecting each memory element to a data source, and a parallel array of bit lines extends in a second direction, connecting each memory element to a data source, the second direction forming an acute angle to the first direction. The connection between each bit line and each memory element is a phase change element composed of memory material having at least two solid phases.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a memory cell known in the art.

FIG. 2 is a top schematic view of an embodiment of a memory array including phase change memory elements.

FIG. 3 is a cross-sectional schematic view of embodiment depicted in FIG. 2.

FIG. 4 is a cross-sectional schematic view of an alternative embodiment of a memory array including phase change memory elements.

FIGS. 5 a and 5 b illustrate the operation of the embodiment depicted in FIG. 1.

FIG. 6 illustrates an aspect of the embodiment of FIGS. 3 and 4.

DETAILED DESCRIPTION

A detailed description of a memory array employing phase change memory cells, and methods for manufacturing such memory cells and arrays, is provided with reference to FIGS. 1-4. It will be understood that embodiments discussed herein and depicted in the drawings are set out for illustrative and explanatory purposes only. It is also expected that alternative embodiments and variations of embodiments will occur to those of skill in the art, in light of the explanations herein. Those and still further alternatives can fall within the scope of the present invention, which is defined solely by the claims appended hereto.

Understanding of the present application will be facilitated by consideration of known techniques for fabricating phase change memory cells. State of the art methods in this field are disclosed and taught in U.S. patent application Ser. No. 11/155,067 entitled “Thin Film Fuse Phase Change RAM and Manufacturing Method”, filed Jun. 17, 2005, and owned by the assignee hereof, which application is hereby incorporated by reference herein. FIG. 1 illustrates the basic memory cell structure 900 taught in that disclosure. It should be noted that the memory cell 900 includes memory elements 900 a and 900 b, each of which stores a bit of data, as known in the art. As seen there, the cell is fabricated on a substrate 902 in which active areas 904 and 906 are formed using known techniques. A common source line 908 provides power, and word line 910 acts as the gate of the MOS transistor thus formed by source 906 and drain 904. A plug electrode 912 connects drain 904 to a pair of spaced electrodes 914 and 918, which electrodes are joined by a bridge of phase change material 916. Another plug electrode 920 connects the second electrode 918 to bit line 922. As is known in the art, a memory array is formed by arraying many such cells, and in a memory array the word lines 910 and bit lines 922 form grids, with the two sets of connectors perpendicularly to one another. In addition, it should also be noted that the disclosed structure occupies three levels, with the transistors lying below the first metal layer (which includes electrodes 914 and 918) and the second metal layer, which includes the bit lines 922.

An embodiment of the claims herein is shown in FIG. 2 (in a top schematic view) and FIG. 3 (a cross-section on plane A-A). This embodiment discloses memory array 10, incorporating memory cells 11, each of which in turn includes memory elements 11 a and 11 b. Each memory element includes active areas 16 and 17 formed on a substrate 14, with source line 20 immediately above the source areas 17 and word lines 18 between the source areas 17 and drain areas 16. Source lines and word lines are all mutually parallel, as discussed above. The bit lines 24, however, are not perpendicular to the word lines. Instead, the bit lines lay at an angle B to the word lines, connected to drain electrodes 22 by phase change memory elements 26.

Embodiments of the memory cell include phase change based memory materials, including chalcogenide based materials and other materials, for the elements 26. Chalcogens include any of the four elements oxygen (O), sulfur (S), selenium (Se), and tellurium (Te), forming part of group VI of the periodic table. Chalcogenides comprise compounds of a chalcogen with a more electropositive element or radical. Chalcogenide alloys comprise combinations of chalcogenides with other materials such as transition metals. A chalcogenide alloy usually contains one or more elements from column six of the periodic table of elements, such as germanium (Ge) and tin (Sn). Often, chalcogenide alloys include combinations including one or more of antimony (Sb), gallium (Ga), indium (In), and silver (Ag). Many phase change based memory materials have been described in technical literature, including alloys of: Ga/Sb, In/Sb, In/Se, Sb/Te, Ge/Te, Ge/Sb/Te, In/Sb/Te, Ga/Se/Te, Sn/Sb/Te, In/Sb/Ge, Ag/In/Sb/Te, Ge/Sn/Sb/Te, Ge/Sb/Se/Te and Te/Ge/Sb/S. In the family of Ge/Sb/Te alloys, a wide range of alloy compositions may be workable. The compositions can be characterized as Te_(a)Ge_(b)Sb_(100−(a+b)). One researcher has described the most useful alloys as having an average concentration of Te in the deposited materials well below 70%, typically below about 60% and ranged in general from as low as about 23% up to about 58% Te and most preferably about 48% to 58% Te. Concentrations of Ge were above about 5% and ranged from a low of about 8% to about 30% average in the material, remaining generally below 50%. Most preferably, concentrations of Ge ranged from about 8% to about 40%. The remainder of the principal constituent elements in this composition was Sb. These percentages are atomic percentages that total 100% of the atoms of the constituent elements. (Ovshinsky '112 patent, cols. 10-11.) Particular alloys evaluated by another researcher include Ge₂Sb₂Te₅, GeSb₂Te₄ and GeSb₄Te₇. (Noboru Yamada, “Potential of Ge—Sb—Te Phase-Change Optical Disks for High-Data-Rate Recording”, SPIE v.3109, pp. 28-37 (1997).) More generally, a transition metal such as chromium (Cr), iron (Fe), nickel (Ni), niobium (Nb), palladium (Pd), platinum (Pt) and mixtures or alloys thereof may be combined with Ge/Sb/Te to form a phase change alloy that has programmable resistive properties. Specific examples of memory materials that may be useful are given in Ovshinsky '112 at columns 11-13, which examples are hereby incorporated by reference.

Phase change alloys are capable of being switched between a first structural state in which the material is in a generally amorphous solid phase, and a second structural state in which the material is in a generally crystalline solid phase in its local order in the active channel region of the cell. These alloys are at least bistable. The term amorphous is used to refer to a relatively less ordered structure, more disordered than a single crystal, which has the detectable characteristics such as higher electrical resistivity than the crystalline phase. The term crystalline is used to refer to a relatively more ordered structure, more ordered than in an amorphous structure, which has detectable characteristics such as lower electrical resistivity than the amorphous phase. Typically, phase change materials may be electrically switched between different detectable states of local order across the spectrum between completely amorphous and completely crystalline states. Other material characteristics affected by the change between amorphous and crystalline phases include atomic order, free electron density and activation energy. The material may be switched either into different solid phases or into mixtures of two or more solid phases, providing a gray scale between completely amorphous and completely crystalline states. The electrical properties in the material may vary accordingly.

Phase change alloys can be changed from one phase state to another by application of electrical pulses. It has been observed that a shorter, higher amplitude pulse tends to change the phase change material to a generally amorphous state. A longer, lower amplitude pulse tends to change the phase change material to a generally crystalline state. The energy in a shorter, higher amplitude pulse is high enough to allow for bonds of the crystalline structure to be broken and short enough to prevent the atoms from realigning into a crystalline state. Appropriate profiles for pulses can be determined, without undue experimentation, specifically adapted to a particular phase change alloy. In following sections of the disclosure, the phase change material is referred to as GST, and it will be understood that other types of phase change materials can be used. A material useful for implementation of a PCRAM described herein is Ge₂Sb₂Te₅.

Other programmable resistive memory materials may be used in other embodiments of the invention, including N₂ doped GST, GexSby, or other material that uses different crystal phase changes to determine resistance; PixCayMnO3, PrSrMnO, ZrO_(x), TiO_(x), NiO_(x), WO_(x), doped SrTiO₃ or other material that uses an electrical pulse to change the resistance state; TCNQ, PCBM, TCNQ-PCBM, Cu-TCNQ, Ag-TCNQ, C60-TCNQ, TCNQ doped with other metal, or any other polymer material that has bistable or multi-stable resistance state controlled by an electrical pulse.

FIG. 3, a cross sectional view of a single memory element 11 a, shows further details of the embodiment shown in FIG. 2. Here, the insulating material 28 that surrounds and encases the elements is shown; it will be understood that this material is likewise present in the embodiments shown in FIGS. 1 and 2 as well, but was omitted for clarity. The insulating material preferably comprises silicon dioxide, a polyimide, silicon nitride or other dielectric fill materials. In embodiments, the fill layer comprises a relatively good insulator for heat as well as for electricity, providing thermal and electrical isolation for the bridges.

This view omits the active areas, likewise for clarity, but word line 18 and source line 20 can be seen, with source plug 19. A plug 13 connects to the drain (not shown), and that plug is connected to the drain electrode 22 by metal deposited in via 15. Bit line 24 lies on the same level as drain electrode 22, and those elements are joined by phase change element 26. The plug elements and source lines preferably are formed from a refractory metal such as tungsten. Other refractory metals include Ti, Mo, Al, Ta, Cu, Pt, Ir, La, Ni, and Ru, as well as oxides and nitrides of such materials. For example, materials such as TiN, RuO or NiO are known and effective refractory metals. The word lines are preferably formed from polysilicon or a suitable silicide, as known in the art.

It is immediately apparent that the structure shown in FIGS. 2 and 3 differs significantly from that of FIG. 1. Conventional technique requires the bit lines to lie perpendicular to the word lines, and thus the bit lines must be formed on a different level than the drain electrodes. Here, reorientation of the bit lines allows them to be formed at the same level as the drain electrodes, so that the phase change elements can join the electrodes and the bit lines directly. The structure of the present application can thus be fabricated with considerably less cost and time than heretofore required.

It is conventional to refer to semiconductor layers in which connection paths are established as “metal” layers, each of which is fabricated in its own set of process steps. The previous state of the art, as exemplified by FIG. 1, requires at least three metal layers to fabricate a phase change memory element. The embodiment of FIGS. 2 and 3 accomplishes that goal with only two, labeled Metal 1 and Metal 2 (100 and 200). For this structure, copper metallization can be used. Other types of metallization, including aluminum, titanium nitride, and tungsten based materials can be utilized as well. Also, non-metal conductive material such as doped polysilicon can be used.

An alternative embodiment is shown in FIG. 4, in which it is desired to employ an extra metal layer labeled Metal 3 (300) between substrate 14 and the electrode/phase change element/bit line layer. Such a manufacturing decision could result from a need to accommodate adjacent structures on the chip, for example, or it could be necessary to position the bit lines and word lines as shown to match the required height of those elements where they interface with other components on the chip. In any event, this embodiment still requires fewer fabrication steps than would an equivalent structure of the prior art, as the equivalent prior art structure would require two layers to accommodate the electrode 22, phase change element 26 and the bit lines 24, leaving the user to deal with a total of four metal layers rather than three. Otherwise, the metallization process follows normal procedures, and the phase change element is formed as described above.

Operation of the embodiments of FIGS. 3 and 4 is shown in FIGS. 5 a and 5 b. As can be seen, the phase change element 26 forms and electrical path between the electrode 22 and the bit line 24. The phase change element thickness (thickness here being taken as the measurement in the vertical dimension of the figure) can be very small in embodiments of the memory cell. This element thickness can be established using a thin film deposition technique on the top surfaces of the electrode 22, insulating material 28, and bit line 24. Preferably, embodiments of the memory cell have a phase change element thickness of about 50 nm or less. Other embodiments of the memory cell have a phase change element thickness of about 05 nm or less. In yet other embodiments, the phase change element thickness is about 10 nm or less. It will be understood that the phase change element thickness can be even smaller than 10 nm, using thin film deposition techniques such as atomic layer deposition and the like, according to the needs of the particular application, so long as the thickness is sufficient for the phase change element performs its purpose as memory element, having at least two solid phases, reversible by a current or by a voltage applied across the first and second electrodes.

Application of a voltage between the electrode 22 and bit line 24 will cause current to flow in the phase change element, and sufficiently high current levels result in joule heating within that element, particularly in area 27 shown in FIG. 5 b. Current levels are chosen to produce an amorphous or crystalline state in the element, as explained above. Alternatively, a low current can be applied for the purpose of reading the data bit stored in the phase change element.

As noted above, two phase change memory elements, such as elements 11 a and 11 b, make up a phase change memory cell. FIG. 6 illustrates in detail the manner in which the bit lines and electrodes of the embodiments described above are positioned to achieve that result. As shown, the electrodes 22 a and 22 b are in electrical contact with the memory cell circuitry, as indicated by the arrows I. Preferably, each of the electrodes is located directly above the active area with which it is in contact, as shown in FIG. 3. Bit line 24 is positioned diagonally with respect to a line between the two electrodes, with each phase change element (26 a, 26 b) joining an electrode and a bit line, thus allowing the bit lines and the electrodes to lie on the same level, as shown.

While the present invention is disclosed by reference to the preferred embodiments and examples detailed above, it is to be understood that these examples are intended in an illustrative rather than in a limiting sense. It is contemplated that modifications and combinations will occur to those skilled in the art, which modifications and combinations will be within the spirit of the invention and the scope of the following claims. 

1. A memory device, comprising: an array of memory elements, formed on a semiconductor chip; a parallel array of word lines, extending in a first direction, connecting each memory element to a data source; a parallel array of bit lines, extending in a second direction, connecting each memory element to a data source, the second direction forming an acute angle to the first direction; and wherein the connection between each bit line and each memory element is a phase change element composed of memory material having at least two solid phases.
 2. The memory device of claim 1, wherein each memory element includes a drain electrode; the phase change element connects a bit line to a memory element at the drain electrode; and wherein the bit line, the phase change element and the drain electrode all lie on the same level within the semiconductor chip.
 3. The device of claim 1, wherein the memory material comprises a combination of Ge, Sb, and Te.
 4. The memory device of claim 1, wherein the phase-change cell comprises a combination of two or more materials from the group of Ge, Sb, Te, Se, In, Ti, Ga, Bi, Sn, Cu, Pd, Pb, Ag, S, and Au.
 5. The memory device of claim 1, wherein the phase-change cell comprises a combination of two or more materials from the group of Ge, Sb, Te, Se, In, Ti, Ga, Bi, Sn, Cu, Pd, Pb, Ag, S, and Au.
 6. The memory device of claim 1, wherein the phase change elements lie above the bit lines in the memory element.
 7. The memory device of claim 1, wherein the memory element includes a plurality of successively formed metal layers, and wherein the phase change elements lie above all such metal layers.
 8. The memory device of claim 7, wherein there are two metal layers.
 9. The memory device of claim 7, wherein there are three metal layers.
 10. The memory device of claim 1, wherein the phase change element has a thickness between 5 and 50 nm.
 11. The memory device of claim 1, wherein the phase change element has a thickness of less than 10 nm.
 12. The memory device of claim 1, wherein the phase change element has a thickness between 0.5 and 5 nm. 